A major sector of the paper industry is directed at the production of paper with good characteristics of liquid absorption and softness, for the production of products such as toilet paper, paper wipes, and the like. This type of paper product goes by the technical name of “tissue paper” and in certain cases “crepe paper”, as a result of the fact that a crinkling or creping is imparted thereon in a step of formation, adopting various possible systems. The most widespread of these envisages the adhesion of the web of cellulose fibers, which still contains a large amount of water, on an internally heated roller or drum of large diameter, referred to as “Yankee drier” or “Yankee roller”. Consequently, upon drying, the fiber web remains adherent to the roller and is detached therefrom using a doctor blade, which impresses a corrugation or crinkling on the paper during detachment thereof from the roller. This corrugation is responsible for an increase in the volume or bulk of the paper and its elasticity, which is prevalently in the working direction of the machine or machine direction, i.e., the direction parallel to the direction of feed of the web through the machine.
Examples of systems for wet production of tissue paper using the above system are described in the U.S. Pat. Nos. 4,356,059; 4,849,054; 5,690,788; 6,077,590; 6,348,131; 6,455,129; 5,048,589; 6,171,442; 5,932,068; 5,656,132; and 5,607,551, and in the European patent No. 0342646.
These systems, referred to technically as “continuous machines”, all envisage, in addition to other elements or particular apparatuses, the presence of a headbox, which forms, on a forming fabric, a layer of a mixture of papermaking fibers and water, with a very low percentage of dry content, in the range of 0.5 wt % to 0.8 wt %. By means of successive steps, the percentage of water is progressively reduced until a web is formed with a dry content of fibers in the range of 48-52 wt %, according to the type of system, at the moment in which the web is transferred from a fabric or felt to the rotating surface of the Yankee roller with the aid of a press, and here the humidity of the web is further reduced, until a percentage of fiber of 95-98 wt % is obtained. The web is at this point considered dry and ready for the next step; consequently, it is detached by the creping blade and then wound on a reel, as mentioned above.
In some systems, such as for example the one described in the U.S. Pat. No. 4,356,059, there are provided two Yankee rollers arranged in series, set between which is a hot-air drying system referred to as “Through Air Drier” (TAD), in which the web of cellulose fibers is entrained around a rotating roller with a pervious cylindrical wall, through which a flow of hot air is generated. This drying system yields a web of large thickness and volume.
The use of the creping blade involves numerous drawbacks, in the first place, tearing of the web. The mechanical action of the blade on the web of fiber is, in fact, rather violent and constitutes the principal cause of tearing of the web during its detachment from the drying roller. Tearing of the web in systems for wet production of paper represents a serious problem in so far as, since it is not possible to stop the system, which is built for working continuously on three shifts a day on account of the thermal inertia especially of the Yankee roller, there will be interruptions in the web wound in reels which entail serious technical consequences and, above all, consequences of an economic nature in the form of a major loss in efficiency of the conversion systems that use these reels.
Other drawbacks of the technique of creping using a blade which cooperates with the Yankee roller are represented by: the fast wear of the creping blade, which must be replaced even twice in a single shift; the high degree of compactness of the fibers in the web that is consolidated and dried on the smooth surface of the Yankee roller; the formation of dense hydrogen bonds between the fibers, oriented prevalently according to horizontal planes; and the difference in thickness of the web obtained with a new blade and with a worn blade, which evidently does not guarantee constancy of characteristics of the web (see U.S. Pat. No. 6,187,137).
According to a different technique, a corrugation in the web is obtained by passing the web still having a high content of humidity from one forming fabric, which moves at a first speed of advance, to a second forming fabric, which moves at a second speed of advance, lower than the first speed of advance. The deceleration undergone by the web causes creping and corrugation thereof. A suction system set appropriately with respect to the forming fabrics withholds the paper material being formed to facilitate the generation of crinkles in the web. Examples of systems based upon this technology are described in the U.S. Pat. Nos. 4,072,557 and 4,440,597.
The U.S. Pat. No. 4,551,199 describes a method and a system in which the web is transferred from a faster fabric to a slower fabric and in which the slower fabric has a particular surface mesh to bring about corrugation of the web.
Similar systems and methods of this type are described in the U.S. Pat. Nos. 5,607,551; 5,656,132; 5,667,636; 5,672,248; 5,746,887; 5,772,845; 5,888,347; and 6,171,442.
In the systems known from these prior art documents, downstream of the fabric on which the corrugation takes place, the web is dried with a TAD system, thus preventing also the other drawbacks linked to the use of the Yankee roller.
On the other hand, the TAD systems are also affected by drawbacks which render their use as an alternative to the drying system with the Yankee drier not always practicable or desirable. For example, the costs in terms of energy consumption are higher, on account of the need to generate enormous rates of flow of hot air that traverses the web to dry it. In addition, the web thus formed is thicker than the one obtained with the creping blade and can present through holes, due to the use of the flow of air that traverses the web to dry it.
To increase the thickness of the paper material produced by continuous machines, there have been suggested various methods and techniques combined with one or the other of the different creping systems. In U.S. Pat. No. 6,077,590, for example, downstream of the Yankee roller with corresponding creping blade there is provided a moistening system or humidifier, in which the paper that has previously been dried and creped is once again moistened. At output from the humidifier, there is provided a wet-embossing assembly, comprising a pair of embossing rollers made of steel, one of which has protuberances and the other has mutually corresponding cavities. The purpose of this system is to obtain a product having a large thickness and a high degree of strength. The use of a Yankee roller and, downstream thereof, of a moistening section and a wet-embossing section involves numerous drawbacks. The main drawbacks are the following: the problems deriving from the risks of tearing of the web on account of the use of a creping blade are not solved; the production line is complex, costly and cumbersome; and the humidification of the web involves high consumption levels in terms of energy and water.
Described in U.S. Pat. No. 4,849,054 is a system in which the web of cellulose fibers with high water content is transferred along its path to a forming fabric that has a surface texture given by the mesh of the fabric that forms it, which imparts an embossing on the web. This is due to the fact that the web, with high water content and hence limited strength, comes to rest on the depressions formed between the threads defining the structure of the fabric. Embossing is facilitated by the use of a suction system set on the side of the fabric opposite to the side on which the web comes to rest. Also in this case, the web embossed using this technique is subsequently dried on a Yankee roller and creped with a creping blade that detaches it from the drying roller. The system is thus characterized by the drawbacks described above, which are linked to the use of creping blades.
The use of a fabric with a surface structure designed to bestow a wet-embossing effect on the web being formed is described also in U.S. Pat. No. 6,187,137 and in WO-A-9923300. Embossing is obtained by the combination of the particular fabric with the aforesaid surface structure by means of a pressurized-air system, which transfers the web from a fabric set upstream to the surface-structured fabric. In order to avoid the use of a creping blade in combination with a Yankee roller and at the same time in order not to use a TAD drying system, with the corresponding costs associated thereto and mentioned above, it has been suggested in the above documents of the prior art to carry out an operation subsequent to embossing on fabric, consisting in making the web, whilst still damp, adhere to a Yankee roller, drying it, and subsequently detaching it therefrom without the use of a creping blade. In this way, drying involves lower costs as compared to drying using TAD systems, and the creping blade, which presents drawbacks deriving therefrom, is not used.
However, this technique involves application on the Yankee roller of a mixture of adhesive agents and of detaching agents in order to enable, on the one hand, proper adhesion of the web to the roller and, on the other, ease of detachment without any risk of tearing and without the use of mechanical members such as the creping blade. The use of this mixture of products, on the one hand, involves drawbacks in terms of consumption and of operating costs and, on the other, constitutes a critical aspect of the process, in so far as the products applied must in effect perform two mutually contrasting actions, with the consequent need to select carefully the products of the mixture and to balance them in a precise and accurate way.
Described in the documents No. US-2002/0060034, US-2002/0124978, and US-2003/0116292 are systems and methods for embossing a layer of tissue paper in conditions of high humidity content. These methods and devices envisage entraining the layer of cellulose fibers around a drying drum provided with protuberances, which impress an embossing pattern on the paper during drying. The paper is pressed against the drying drum provided with protuberances via a fabric or felt set behind which is a pressure roller, or else directly via a pressure roller made of compliant material.